Describes the activity of adrenochrome and some of its derived compounds. The effect of adrenochrome is considered first upon simple systems, then more complex systems, then simple animals, and finally, upon the most complex animal, the man. animals that are given adrenochrome range from spiders, fish, and pigeons to the mammals including rats, cats, dogs, monkeys, and man. The chapter gives much original data on cats and man. Chemically, adrenochrome and adrenolutin are very reactive substances. Some of the changes produced by adrenochrome may persist several days, and in some cases, the effects lead to nearly disastrous results. Two cases of prolonged reactions are discussed in the chapter. There is also a discussion regarding the reaction that lasted more than one day after a single administration of adrenolutin as well as reactions up to one week. The changes in thinking induced by adrenochrome are similar to those observed in schizophrenia. Adrenochrome causes an elective inhibition of the process, which determines the content of associative thinking. This occurs in doses that do not heighten the lability of basic processes, do not reduce excitation, and do not loose temporary connections as is the case with LSD.